Initially, the first generation of superconductors included materials that exhibited superconducting properties at temperatures only below about 20K. These were known as low temperature superconductors. New, second-generation high temperature superconductors (HTS) are now being developed. These second-generation HTS are resistance-free conductors made of ceramic materials that exhibit superconducting properties at temperatures between about 25-130 K, which therefore require less expensive cooling systems than those needed for low temperature superconductors.
Several commercial electric power and military applications would benefit from the use of second-generation high temperature superconductors. However, there are presently several hurdles that need to be overcome before high temperature superconductors can be successfully utilized in such applications. One obstacle that needs to be overcome is that the superconducting material must be able to operate in the high magnetic fields that such devices operate at. The magnetic field that the superconducting material will experience in many such devices (i.e., commercial generators and motors, and most military applications) may be in the range of 1-5 Tesla. In a typical coated superconductor tape, when a magnetic field of 1 Tesla is applied perpendicular to the tape's surface, the critical current density decreases by a factor of about seven to about ten from its self-field value. As a result, in order to achieve 100 A/cm in a magnetic field of 1 Tesla at 77K, a critical current density of 1 MA/cm2 needs to be achieved in a 1 micron thick film at 1 Tesla and 77K. Such high levels of critical current densities have not yet been demonstrated, not even in short lengths of coated superconducting tape.
In attempts to increase the performance of high temperature superconductors in high magnetic fields, some present high temperature superconductor prototype devices are being cooled to about 30K. However, cooling these devices to this low temperature increases the operational costs of the superconductor, and decreases the reliability thereof.
Other attempts to increase the performance of high temperature superconductors in high magnetic fields involve utilizing thick films. However, making thick films is a complex and expensive process, and only limited success has been achieved by utilizing this technique. Essentially, to achieve 100 A/cm at 1 Tesla and 77K, a critical current of 700 to 1000 A/cm width has to be achieved at zero applied field and 77K, and this level of performance has not yet been achieved in thick films. Furthermore, increasing the film thickness reduces manufacturing throughput and increases material costs.
Therefore, there is a need for high temperature superconductors that can operate at high temperatures and high magnetic fields. Such high performance HTS materials would ideally comprise rare-earth-Ba—Cu—O coated superconductors. It would be desirable to have such HTS materials comprise yttrium (Y) and/or heavy rare earth materials such as samarium (Sm), ytterbium (Yb), neodymium (Nd), gadolinium (Gd), europium (Eu), lanthanum (La), dysprosium (Dy), holmium (Ho), and/or mixtures thereof. It would be desirable to have such HTS materials have a superior critical current density. Ideally, these HTS would possess superior performance in the presence of a magnetic field (i.e., achieve 100 A/cm at 1 Tesla and 77K). It would be further desirable to have such HTS materials have minimal degradation of Jc when a magnetic field is applied normal to the surface of the HTS. For example, it would be desirable to have such HTS materials have a drop in Jc of less than a factor of about 7 at a temperature of about 30-77K, and at a magnetic field of about 1 Tesla, when the magnetic field is applied normal to the surface of the HTS. It would be yet further desirable to have such HTS materials have a peak Jc when a magnetic field is applied perpendicular to the surface of the HTS that is at least about 50% of the peak Jc that exists when the magnetic field is applied parallel to the surface of the HTS. It would be even further desirable to have such HTS materials have a Jc value when a magnetic field is applied in any orientation with respect to the HTS surface that is at least about 50% of the peak Jc that exists when the magnetic field is applied parallel to the surface of the HTS. It would be yet further desirable to have such HTS materials comprise kilometer lengths of metal tape as the substrate. It would be still further desirable to be able to make such HTS materials in a manner that allows suitably high manufacturing rates to be achieved. Many other needs will also be met by this invention, as will become more apparent throughout the remainder of the disclosure that follows.